1. Field of the Invention
This invention relates to control systems for synchronous machines, and more particularly, to an under-excitation limiter for a microprocessor based automatic voltage regulator for a synchronous generator connected to a power grid system.
2. Background Information
The increased growth of power generation and distribution systems requires improved methods of controlling power system electrical quantities. System stability becomes more and more important as the systems grow larger. One of the major determinants of system stability is automatic voltage regulation equipment for synchronous generators. The primary function of this equipment is to control the individual machine terminal voltage. The modern voltage regulator must be more than a regulator of voltage. It must be supplemented by other controls that directly affect system stability; such as the Minimum Excitation Limiter (MEL).
MEL equipment is typically used with, and made part of, the voltage regulation equipment for synchronous generators. The function of the minimum excitation limiting equipment is to maintain the excitation of a synchronous machine above a predetermined MVA characteristic. This strategy will prevent such a machine from reaching an under-excited condition that would endanger its synchronization with the power system, and adversely affect overall system stability. The MEL acts through the voltage regulator to manipulate machine excitation.
In order for a synchronous machine to maintain synchronism with the balance of the system, certain relationships must exist between terminal voltage, real power and reactive power. This relationship is termed the "static stability limit" (SSL) of the machine. The common model of a synchronous generator connected to a power system characterizes the static stability limit as a sector of a circle in the real/reactive power plane.
A second relationship relates to the safe operation of the generator itself, rather than to the stability of the generator connected to the system. It is known as the generator capability curve. This curve establishes limits to protect the machine from thermal damage. Actually, a family of curves dependent on cooling effectiveness can be generated for a given machine. Any curve from this family when plotted in the real/reactive power plane is substantially different from, but generally intersects, the SSL curve. The machine capability curves are used in the design of synchronous generators, but to our knowledge are not used to influence the control of a synchronous machine in real time.
A common minimum excitation limiter for synchronous generators is an analog device having a response characteristic which correlates fairly well with the SSL curve. These limiters do not correlate well at all with, and in fact, do not even take into account, the machine capability curves.
Synchronous generators are also protected by a loss of field relay which disconnects the generator from the grid if the field current is very low. Generally, the operating characteristic for the loss of field relay is a straight line in the real/reactive power plane, but outside the limits set by the SSL. However, in some applications, where the SSL is not a real concern, it could be that a utility would want to employ the minimum excitation limiter to correlate with the loss of field relay and raise the excitation to avoid unnecessary trips with the loss of field relay. As the SSL is characterized as an arc of a circle, it does not correlate well with the straight line loss of field relay characteristic.
There is a need therefore for improved protection for synchronous generators from under-excitation.
There is a further need for apparatus which provides more flexible protection, and more particularly which allows for selective better correlation with one or more protection characteristics.
There is also a need for such improved apparatus which can accommodate for changes in operating characteristics of the synchronous generator, such as changes in the cooling effectiveness.
Specifically, there is a need for such improved apparatus which allows a utility the opportunity to provide protection for a synchronous generator which correlates well with the machine capability limits.
There is a further need for such apparatus which allows a utility the opportunity to provide protection for a synchronous generator which correlates well with a loss of field relay.